Two stage collapse using rigid body physics?
Unfortunately blender doesn't really have much support for this kind of thing at the moment (the rigid body simulator itself was just added fairly recently). Ideally the fracturing would be done real-time, based on impact forces etc.

However, it's possible to fake it with constraints, as you mentioned.
I like use this addon for setting up lots of breakable constraints between the shards.

There are a couple ways you can go about this.

One way is to fracture object object into lots and lots of little shards, and constrain everything together with breakable constraints (using the addon).
Video of result, and blendfile.



Another way is to fracture the object normally, then manually cell-fracture the bigger shards and constrain them together with the addon.
Video of result, and blendfile